1. Field of the Invention
The present invention relates to a magnetic tunnel junction device having a layered structure of ferromagnetic layer/insulating layer/ferromagnetic layer, in which an electrical resistance value between the ferromagnetic layers via the insulating layer changes depending on the relationship between the magnetization directions of the ferromagnetic layers that sandwich the insulating layer.
2. Description of Related Art
A magnetic tunnel junction device having a layered structure in which an insulating layer is sandwiched between a pair of ferromagnetic layers is known. In the magnetic tunnel junction device, an electrical resistance value between the pair of ferromagnetic layers via the insulating layer changes depending on the relationship between the magnetization directions of the ferromagnetic layers that sandwich the insulating layer. This change in resistance value is referred to as magnetoresistance or magnetoresistance change. The magnetic tunnel junction device has a structure in which, for example, one (a pinned layer) of the ferromagnetic layers that sandwich the insulating layer has a fixed magnetization direction and the other ferromagnetic layer (a free layer) has a magnetization direction that can be changed. In this structure, the electrical resistance value between the pinned layer and the free layer changes when the magnetization direction of the free layer changes and the relationship between the magnetization directions of the pinned layer and the free layer changes from parallel to antiparallel and from antiparallel to parallel.
As an insulating layer for the magnetic tunnel junction device, a magnesium oxide (MgO) layer and a magnesium fluoride (MgF2) layer are known (see JP 2001-156357 A).
S. Mitani et al., “Structure and tunnel magnetoresistance in Fe/MgF2/Co junctions with an oxide seed layer on an Fe bottom electrode”, Journal of Applied Physics, vol. 91 (2002), pp. 7200-7202 shows that a MgF2 insulating layer with reduced pinholes and reduced leakage current is formed by inserting a MgO seed layer. FIG. 14 shows the structure of a magnetic tunnel junction device disclosed in Mitani et al. A device 601 shown in FIG. 14 has a layered structure of Fe layer 603/MgO layer 604/MgF2 layer 605/Co layer 606 on a substrate 602. The MgO layer 604 is a seed layer for forming the MgF2 layer 605 on the Fe layer 603. The MgO layer 604 is a separate layer from the MgF2 layer 605, and thinner than the MgF2 layer 605. However, the magnetic tunnel junction device of Mitani et al. shows a magnetoresistance ratio of less than 1% at room temperature and 10% at a temperature of 4.2 K. The performance of the device of Mitani et al. is significantly lower than that of a conventional magnetic tunnel junction device with a magnetoresistance ratio of several tens of percent or more at room temperature.
J. H. Kwon et al., “Effect of F-inclusion in nm-thick MgO tunnel barrier”, Current Applied Physics, vol. 9 (2009), pp. 788-791 discloses a technique for forming a modified insulating layer made of oxyfluoride by oxidizing an aluminum metal layer or a magnesium metal layer in fluorine-containing oxygen. FIG. 15A and FIG. 15B show FIG. 5 of Kwon et al. FIG. 15A and FIG. 15B each show a barrier height for tunneling current (barrier height for tunneling electrons) in an oxyfluoride insulating layer disclosed in Kwon et al. A barrier height for tunneling current is an electrical evaluation index indicating the quality of an insulating layer (tunnel barrier). FIG. 15A shows an example in which a top electrode and a bottom electrode are made of aluminum (Al), and an insulating layer is a layer (AlOx) obtained by merely oxidizing an aluminum metal layer or a layer (AlOF) obtained by oxidizing an aluminum metal layer in fluorine-containing oxygen. A solid line and a dotted line indicate the barrier height of the AlOx layer and that of the AlOF layer, respectively. FIG. 15B shows an example in which a top electrode and a bottom electrode are made of magnesium (Mg), and an insulating layer is a layer (MgO) obtained by merely oxidizing a magnesium metal layer or a layer (MgOF) obtained by oxidizing a magnesium metal layer in fluorine-containing oxygen. A solid line and a dotted line indicate the barrier height of the MgO layer and that of the MgOF layer, respectively.
As shown in FIG. 15A, the AlOF layer has a higher barrier height than the AlOx layer. On the other hand, as shown in FIG. 15B, there is no difference in the barrier height between the MgOF layer and the MgO layer. That is, Kwon et al. discloses that the addition of fluorine during oxidation of aluminum increases the barrier height of the resulting insulating layer but no such effect as seen in aluminum is not obtained by adding fluorine during oxidation of magnesium.
Mitsuo Shimbo et al., “Effect of fluoride addition on crystal growth of MgO”, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi (Journal of the Ceramic Society of Japan), vol. 97 (1989), pp. 857-863 discloses the effect of fluoride on the crystal growth of MgO. More specifically, it shows that the crystal growth of MgO is accelerated at temperatures above 900° C. by adding fluoride during sintering of MgO powder into MgO ceramic bulk. FIG. 16 shows FIG. 3 of Shimbo et al. FIG. 16 shows the effect of MgF2 addition on the grain size of sintered MgO. FIG. 16 indicates that the MgO crystal grain size is increased by adding MgF2 at sintering temperatures of 900° C. or higher, but the crystal growth of MgO is rather inhibited by adding MgF2 at temperatures below 900° C.